1. Field of the Invention
The present invention relates to oscillographic apparatus and, more particularly, to a control circuit for such oscillographic apparatus.
2. Description of the Prior Art
In the art relating to oscillographic apparatus, whether it be an oscillographic display device or an oscillographic recording apparatus, a sweep signal is generated to cause the beam of the oscillographic apparatus to traverse across the face of the instrument. In the oscillographic presentation of repetitive wave form input signals, it is desirable to have the sweep signal synchronized with the repetition of the input signal. To that end, a trigger signal is derived from the comparison of an event signal which may be, or is related to, the input signal. It is usual that the trigger signal is derived from a point on the positive or rising slope of the determining signal wave form. When, on occasion, it is desired to carefully analyze the input wave form, the scale of the display is expanded to the point where the time of the sweep signal is less than the period of the input signal, some of the data in the input wave form is lost. On those occasions, it has been found desirable to switch the triggering of the sweep signal from the positive slope of the determining signal to the negative slope, thus, shifting the phase of that portion of the input wave form being presented for analysis. In such systems heretofore, the switching of the input, or determining signal, between the positive and negative slope has introduced two problems. One, the switching of the high-level wave form signals introduces electromagnetic discharge noise. Two, for a fixed reference potential, the system does not respond to provide a trigger signal at the same level on the negative slope as on the positive slope, leading to inaccuracies in the resultant output display, or presentation.